1. Field of the Invention
This invention relates to an internal combustion engine, mounted on a personal watercraft which is designed for operation in water. More particularly, the invention relates to an internal combustion engine having an improved lubricating oil cooling structure.
2. Description of the Background Art
The personal watercraft, or small-sized planing boat, is constructed such that an internal combustion engine, for driving a jet propulsion pump, is mounted in a boat body and enclosed by a hull and a deck. A driver and up to two other crew members ride on the deck, so that an internal cabin space, constituted by the space between the hull and the deck, is narrow. The internal combustion engine is stored in a substantially closed and sealed state within the narrow space between the hull and the deck.
As a consequence, a compact internal combustion engine is required. In order to minimize the height of the internal combustion engine, a dry sump engine has been used, having no significant oil reservoir at the bottom of the engine, but storing the oil in a separate oil tank instead. Such an internal combustion engine is disclosed, for example, in Japanese published patent document No. 2003-35201.
A water-cooled oil cooler is provided for an oil path according to the dry sump lubrication system of JP-A No. 2003-35201, whereby the rise of the temperature of lubricating oil is inhibited. In the disclosed small-sized personal watercraft, cooling water taken from the side of positive pressure of a jet propulsion pump is used for cooling an internal combustion engine and the oil cooler also utilizes the cooling water.
In this cooling system, since cooling water is not circulated between a radiator and the internal combustion engine, and since new cooling water is constantly supplied, the cooling power of the water is high. However, in very cold water conditions, supercooling of the internal combustion engine may occur. During supercooling, fuel in a combustion chamber invades a crankcase from between a cylinder and a piston, is mixed with lubrication oil within the crankcase, so-called dilution occurs. As a result, deterioration of the lubrication oil is accelerated, having a negative effect upon the life of oil.
The invention is made in view of the above described problem. The object of the invention is to provide an internal combustion engine for a personal watercraft in which the use of an oil cooler is controlled, so that supercooling is avoided in very cold water conditions and the dilution of lubricating oil can be prevented.